1. Field of the Invention
This invention relates to a high melt flow copolymer of tetrafluoroethylene and hexafluoropropylene capable of being extruded at high speed.
2. Description of Related Art
U.S. Pat. No. 5,677,404 discloses an improved fluoropolymer, wherein the improvement enables the fluoropolymer to be extruded at high speeds without sacrifice of stress crack resistance. This polymer is successfully extruded on to a conductor to make insulated wire of high quality (fewer than 10 sparks and 2 lumps/13 km of conductor coated) at speeds in excess of 1900 ft/min (579 m/min). The UL 444 industry standard for spark failures is no more than 15 spark failures per 45,000 ft (13.7 km) of coated conductor. A spark failure indicates a fault in the insulation. Industry prefers that no more than 10 spark failures be present/13.7 km of insulated conductor to insure acceptable insulated conductor. An additional quality criterion desired by the industry is that for the same length of coated conductor, the insulation should have no more than 2 lumps/13.7 km. Lumps in the insulation interfere with the ultimate use of the insulated conductor; e.g. twisting together to form twisted pair conductors, pulling the insulated conductor through narrow openings.
Speeds up to 2250 ft/min (686 m/min) can be easily achieved. Higher speeds are possible but non-polymer specific limitations arise. Therefore production of good quality insulated conductor at line speeds of from about 1750 to 2250 ft/min (533 to 686 m/min) is considered excellent performance. However, it has been found that the temperature of the molten polymer in extrusion must be closely controlled to achieve excellent performance. Loss of control results in unacceptably high incidences of insulation faults such as sparks (points at which the polymer inadequately coats the conductor) and lumps (regions of irregular geometry of the insulation). It has further been found that lot-to-lot variations in the fluoropolymer melt flow rate can upset the close control of extrusion and require time-consuming and wasteful adjustments, during which time unsaleable product is made. Reduction in fluoropolymer melt flow rate variation would impose significant economic penalties.
Further polymer improvement is needed to permit high speed extrusion, particularly for extrusion of fluoropolymer insulation with few or no sparks or lumps over a broader temperature range than is now possible.